Does the probabilistic maturation reaction norm approach disentangle phenotypic plasticity from genetic change?

Abstract

Two-dimensional probabilistic maturation reaction norms (PMRNs) define the probability that individuals mature as a function of age and size. PMRNs have recently been used to derive empirical support for the hypothesis that high fishing mortality has induced genetic change towards earlier maturation in exploited populations. However, the 2-dimensional PMRN method does not, strictly speaking, disentangle phenotypic plasticity from genetic change in maturation. Instead, it disentangles the contribution of variation in growth and mortality from other sources of variation contributing to changes in maturation. After removing the contribution of variation in growth and mortality, any remaining change is not necessarily purely genetic. Environmental factors may exist that directly affect the propensity to mature at a certain size and age. An observed trend in the sizeage PMRN may therefore be partly or fully explained by a co-incidental trend in an (environmental) factor. As pointed out by Grift et al. (2007; Mar Ecol Prog Ser 334:213-224), such co-dependence is captured conceptually by multi-dimensional PMRNs, as opposed to 2-dimensional (size-age) PMRNs, where the higher dimensions represent the relevant variables. Dimensions that may partly or fully explain the observed trends in size-age PMRNs, other than evolution, include temperature, polluting endocrine-disrupting chemicals, the social structure of the population, and body condition; changes in the last two may be fisheries-induced. To illustrate these views, I re-analysed data on North Sea plaice, and demonstrated that temperature explains part of the shift in the size-age PMRN but that a residual shift remains. The latter finding supports the hypothesis of genetic change.

Fisheries-induced evolutionary changes in maturation reaction norms in North Sea sole Solea solea

“…The length-based reaction norm is more reliably estimated than the weight-based reaction norm because of the ambiguity in data selection and corresponding interpretation for the weights (see below) resulting in a smaller data set and higher uncertainty in the reaction norm estimates. However, other factors in addition to size, such as body condition (Kjesbu et al 1991, Trippel & Neil 2004 or temperature (Dhillon & Fox 2004, Kraak 2007, may affect the probability of becoming mature.…”

“…It is known that the onset of maturation may be affected by condition (Kjesbu et al 1991, Trippel & Neil 2004 or other factors such as temperature (Dhillon & Fox 2004, Kraak 2007, in the juvenile period 1 or more years prior to first maturity. The influences of average temperature, condition and competitive biomass were analysed on the population level in the years before maturation.…”

“…This growth conditional PMRN has been applied to several stocks supporting a fisheries-induced evolutionary change in the onset of maturity (reviewed in . Although the method has been criticized, as it does not disentangle genetic effects from environmental factors affecting maturation other than through size at age (Kraak 2007, Marshall & McAdam 2007, it can easily be extended to incorporate other variables as a third or higher dimension (Grift et al 2007).…”

Red snapper management in the Gulf of Mexico: science- or faith-based?

“…Those that that are genetically predisposed to mature earlier then have a selective advantage under fishing pressure and a greater chance of contributing spawning products over late-maturing individuals. This ultimately leads to directional selection for early maturing females in the population (Rijnsdorp 1993;Kraak 2007;Law 2007 and papers in theme session entitled Disentangling the causes of maturation trends in exploited fish populations, Mar Ecol Prog Ser. 335 (2007)).…”

Temporal trends in age and size at maturation of four North Sea gadid species: cod, haddock, whiting and Norway pout

“…Likewise, elevated temperature accelerates maturation (Dhillon & Fox 2004, Morita et al 2009). In some fish species, maturation is also influenced by the social and/or size structure of the population (Hutchings et al 1999) that fishing may alter by reducing the population's abundance or truncating its age and size structure: the resulting relaxation of the social pressure exerted by old and/or large dominant individuals may enable younger and smaller ones to mature (Sohn 1977, Hobbs et al 2004, Kraak 2007.…”